A government official left two intelligence documents containing information about al-Qaida and the situation in Iraq on a train in London, it has emerged.
The documents, both reports by the joint intelligence committee, were abandoned on a train and belonged to a top Cabinet Office official. One covered al-Qaida's current weaknesses while the other focused on Iraq.
The official has now been suspended following an emeregency internal inquiry.
A passenger on the train, leaving London's Waterloo station on its way to Surrey, found the papers and handed them to the BBC. The incident appears to be a major breach of security rules governing the handling of top-secret information.
A Cabinet Office spokesperson confirmed that a "security breach" had occurred and added the Metropolitan police was carrying out an investigation to determine the circumstances surrounding the incident.
Opposition politicians have immediately jumped to criticise the incident, which has come under repeated attacks over data security since HM Revenue and Customs lost the personal records of 25 million child benefit claimants in November 2007.
Shadow security minister Baroness Neville-Jones called for an immediate statement to parliament on the issue and demanded an inquiry be launched.
"This is just the latest in a long line of serious breaches of security involving either the loss of data, documents or government lap tops, further highlighting the most basic failures in this government's ability to maintain our security," she said.
Liberal Democrat home affairs spokesperson Chris Huhne went further, describing the incident as "an appalling breach of security".
"There should be strict guidelines about when such secret documents are outside carefully monitored premises," he commented.
"It beggars belief that the government could have scored such a devastating own goal on the very day that it was pushing draconian counter terrorism laws through parliament."